This invention relates to a raster scanning system and, more particularly, to a scanning system utilizing a rotating polygon whose facets are illuminated in an overfilled design and whose edges are coated with a non-reflective coating.
Many prior art raster output scanners (ROS) utilize a rotating polygon having flat reflective surfaces, or facets, in parellel with the axis of rotation of the polygon. In a typical system, a beam is emitted from a light source such as a helium-neon laser. The light is directed through a pre-polygon conditioning optics, modulated according to an input electrical signal, onto the rotation polygon surfaces. The high speed rotation of the polygon, typically in a 3 to 15 krpm range, then scans the beam through a post-polygon conditioning lens and images the laser spot across the full process width of a photosensitive image plane. In these prior art ROS systems, the pre-polygon conditioning optics typically are incorporated in an underfilled facet design; e.g., the light beam directed against the rotating polygon illuminates only a portion of each rotating surface (facet). Overfilled facet designs, where the light beam completely illuminates each facet and a small portion of adjacent facets, have been used to some degree, but have not gained wide acceptance. Comparing the two designs, in an overfilled design the facet size required to produce a given spot size at the image plane is greatly reduced allowing many more facets to be accommodated on the same diameter polygon. This, in turn, permits the scan system to operate it at a relatively low rotation rate permitting the use of less powerful (and less expensive) polygon motor drives. This advantage has, heretofore, been more than offset, by two factors: low throughput efficiency and non-uniform illumination at the polygon facets. In order to tolerate the low efficiency (typically 10 to 15 %), a higher powered laser diode is required. The non-uniformity problem occurs because the light beam directed across the rotating polygon has a Gaussian spot shape which has been expanded so that more than one facet of the polygon is illuminated. As the polygon rotates to scan the spot across an output medium, the amount of light reflected to the medium varies because the facets are sampling different parts of the Gaussian illumination profile and the effective area of the polygon is changing.
Attempts to compensate for this illumination uniformity have taken several forms. One method is to change the spot intensity with scan angle by using a smile correction algorithm which changes the output of the laser diode to maintain uniform exposure at the photoreceptor image plane. Another method, disclosed in co-pending U.S. application Ser. No. 359,604, now U.S. Pat. No. 4,941,721 filed on Jun. 1, 1989, is to introduce an aspherical lens system between the laser source and the rotating polygon. The aspheric lens transforms the non-uniform Gausian light beam profile into a colliminated flat intensity beam directed across the width of the polygon facets, the beam overfilling each facet. With either of these two methods, however, there still remains a residual non-uniformity problem which is inherent in the basic design of any polygon scanner. The mirrored surfaces (facets) are manufactured with a high degree of flatness precision. Typically 1/4 wavelength of light is the tolerance, but the facets cannot be held to this tolerance at the facet edges. Any deviation from the flatness specification near the corner edges of each facet will result in a degradation of the spot imaged at the photoreceptor. The amount of degradation of the spot depends upon the magnitude of departure from flatness and the areas of the facet over which the departure exists.
According to the present invention this facet edge non-uniformity is compensated for by operating in an overfilled design mode and by having the edges between the facets of the polygon coated with a non-reflective coating to eliminate light reflected from the out-of-spec flat edge areas. More particularly the invention relates to a raster scanning system utilizng an overfilled polygon facet design for forming latent images at a photosensitive medium comprising: means for providing a beam of radiant energy, means for modulating said energy beam in response to electrical signals, means for expanding the modulated beam, and a polygon having at least one reflective facet positioned in the optical path of said modulated beam and adapted to scan said spot across said medium, each of said facets characterized by having two corner edges coated with a non-reflective material.
Some relevant prior art disclosures are found in the following patents: U.S. Pat. No. 3,944,323 to Starkweather, assigned to Xerox Corporation, discloses a variable spot size scanning system. A means is provided for covering a portion of the lower half of the facets. This system is intended to provide a more continuous gray scale for a scanner.
U.S. Pat. No. 4,357,071 to Mankel et al. discloses an optical fault seeking apparatus. A polygon mirror is shaped with flat facets and concave corners. The apparatus is used for scanning fabrics for faults in workmanship.